1. Field of the Invention
The invention relates to a liquid crystal display device and a method of fabricating the same, and more particularly to an in-plane switching mode active matrix type liquid crystal display device and a method of fabricating the same.
2. Description of the Related Art
A liquid crystal display device may be grouped into two types, one of which the molecular axes of aligned liquid crystal molecules are rotated in a plane perpendicular to a substrate to display a certain image, and the other of which the molecular axes of aligned liquid crystal molecules are rotated in a plane parallel with a substrate to display a certain image.
A typical one in the former type is a twisted nematic (TN) mode liquid crystal display device, and the latter type is called an in-plane switching (IPS) mode liquid crystal display device.
Since a viewer looks only at a direction in which minor axes of liquid crystal molecules extend, even if he/she moves his/her eye point, in an IPS mode liquid crystal display device, how liquid crystal molecules stand is not dependent on a viewing angle, and accordingly an IPS liquid crystal display device can present a wider viewing angle to a viewer than a TN mode liquid crystal display device.
Hence, an IPS mode liquid crystal display device has been more popular these days than a TN mode liquid crystal display device.
For instance, Japanese Unexamined Patent Publication No. 07-036058 has suggested an example of an IPS mode liquid crystal display device. Attempts have been made to present a higher aperture ratio in an IPS mode liquid crystal display device, for instance, in Japanese Unexamined Patent Publications Nos. 11-119237, 10-186407, 9-236820, and 6-202127.
The IPS mode liquid crystal display device suggested in Japanese Unexamined Patent Publication No. 11-119237 is characterized in that both a drive electrode (which corresponds to the pixel electrode in the present invention) and an opposing electrode (which corresponds to the common electrode in the present invention) are formed in a layer which is different from a layer in which a signal line is formed, and is located closer to a liquid crystal layer. This structure ensures that the opposing electrode is less influenced by an electric field generated due to a voltage difference between the signal line and the opposing electrode formed at an end of an opening and adjacent to the signal line, and hence, it would be possible to locate the opposing electrode closer to the signal line. As a result, it would be possible to increase an area of the opening.
Japanese Unexamined Patent Publication No. 11-119237 further suggests that the drive electrode and the opposing electrode are composed of transparent material such as ITO. However, it is not disclosed that the opposing electrode overlaps the signal line.
Japanese Unexamined Patent Publication No. 11-119237 further suggests an IPS mode liquid crystal display device in which both the drive electrode and the opposing electrode are formed in a layer located above a layer in which the signal line is formed, and the opposing electrode overlaps the signal line. This structure ensures that the drive and opposing electrodes are less influenced by an electric field leaking out of the signal line, and that a light is not leaked out of a slit formed between the signal line and the opposing electrode.
However, there is no suggestion that the drive and opposing electrodes are formed as transparent electrodes, because the opposing electrode is formed to overlap the signal line for the purpose of interrupting a light leaking out of the slit.
In the IPS mode liquid crystal display device disclosed in Japanese Unexamined Patent Publication No. 10-186407, an electrically insulating layer is formed between a common electrode layer of which a common electrode is formed and a data line layer of which a data line is formed, and the common electrode layer is located closer to a liquid crystal layer than the data line layer. The common electrode overlaps a particular area of the data line in a particular area. The common electrode entirely overlaps the data line, preventing leakage of an electric field, and the common electrode partially overlaps the data line, ensuring reduction in a parasitic capacity formed between the data line and the common electrode.
However, Japanese Unexamined Patent Publication No. 10-186407 does not disclose and suggest a common electrode composed of transparent material.
In the IPS mode liquid crystal display device disclosed in Japanese Unexamined Patent Publication No. 9-236820, each of opposing electrodes is comprised of thin strip-shaped electrodes parallel with source bus lines (which correspond to the data lines in the present invention) through which a pixel signal is transmitted to a pixel electrode. The opposing electrodes and the source bus lines are stacked one on another with transparent insulating layers being sandwiched therebetween. The opposing electrodes and the source bus lines are located at the same position with respect to a direction in which a light passes through the liquid crystal display device.
Japanese Unexamined Patent Publication No. 9-236820 describes that it would be possible to increase an aperture ratio of pixels, if the opposing and pixel electrode were composed of transparent material. However, the Publication further describes that since transparent material has a high resistance, a voltage difference would be generated, disturbing driving the electrodes to display images, and that a transparent electrode is quite expensive.
An IPS mode liquid crystal display device disclosed in Japanese Unexamined Patent Publication No. 6-202127 is designed to have a driver comprised of an active device and is characterized in that a signal line through which an image signal is transmitted to the active device is covered in an area facing a liquid crystal layer with an electrical conductor with an electrical insulator being sandwiched therebetween.
However, the Publication never discloses and suggests that a signal line is shielded with a transparent electrode.
An IPS mode liquid crystal display device disclosed in Japanese Unexamined Patent Publication No. 10-307295 is characterized by a plurality of sub-areas for compensating for colored images. As an example, the Publication suggests a method of preventing images from being colored, including the step of generating electric fields having different directions in first and second sub-areas, to thereby rotate liquid crystal molecules in different directions in the first and second sub-areas, ensuring that optical characteristics of the first and second sub-areas are compensated for to each other when a viewer obliquely views the liquid crystal display device.
The IPS mode liquid crystal display devices disclosed in the above-mentioned Publications have an object of increasing an aperture ratio, and enhancing a brightness of displayed images.
Since there is generated a voltage difference between a data line and an opposing electrode or a common electrode, an electric field is generated due to the voltage difference. If the electric field reaches such a region that a display region located between a pixel electrode and a common electrode is influenced by the electric field, alignment in liquid crystal molecules is disturbed. For instance, when a white window is to be displayed on a screen under black background, there would be caused a problem called vertical cross-talk by which pixels which should display black, associated with a data line driving pixels displaying white, display gray.
In order to avoid the vertical cross-talk problem, it would be necessary to terminate the electric field with the common electrode having a width extending outwardly at opposite edges of the data line for shielding an electric filed associated with the data line, or to cover the data line with an electrode to which a voltage exerting no influence on images is applied, such as the common electrode.
In order to increase an aperture ratio, it would be preferable to cover the data line with the common electrode, as mentioned in the latter.
However, the conventional liquid crystal display devices suggested in the above-mentioned Publications are accompanied with problems of incomplete shield shielding and reduction in an efficiency with which a light is used which reduction is caused by a common electrode composed of light-impermeable material.
The conventional liquid crystal display devices suggested in the above-mentioned Publications have an object of enhancing an aperture ratio, however, there is a need of further enhancement of an aperture ratio.